1. Field of the Invention
The present application relates to a positive temperature coefficient (PTC) device of which the resistance increases as temperature rises. The PTC device is suitably applied to temperature sensors and over-current protection.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Because the resistance of conductive composite materials having positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the crystalline polymer of the PTC conductive composite material will melt and expand to sever a lot of conductive paths and therefore the resistance instantaneously increases to a high resistance state (i.e., trip) to decrease the current.
It is highly demanded for over-current and over-temperature protections the PTC device has low resistance at room temperature, large resistance variation between room temperature and trip temperature and superior resistance repeatability after repetitive trips.
The PTC device usually uses carbon-series conductive fillers such as carbon black and graphite; however it is necessary to use a lot of carbon-series conductive fillers to meet low resistance requirement. As a result, the PTC device may not trip if a large amount of carbon black are used and therefore it may be not qualified for over-current or over-temperature protection.
This shortcoming may be overcome by using metal conductive fillers, e.g., nickel, of which the resistance is much lower than that of the carbon-series fillers. However, it was observed that the resistance of the PTC device at room temperature gradually increases over time, and therefore the reliability for long-term operation is not satisfactory. It is believed that the metal conductive fillers would be oxidized and thus the electrical conductivity becomes lower.
U.S. Pat. No. 6,778,062 discloses an organic PTC thermistor comprising an organic polymer matrix and conductive metal particles dispersed therein. The conductive metal particles are pretreated with an organic material. The organic material is different from the organic polymer matrix, does not covalently bond with the conductive metal particles, and is not compatible at a molecular level with the organic polymer matrix, so that an organic material layer covers surfaces of conductive metal particles to avoid oxidation whereby resistance stability is improved. However, the conductive metal particles have to be pretreated in advance, and therefore the process becomes more complicated. Furthermore, the process stability and quality of the conductive metal particles would be hard to control, and these problems affect the electrical performance of the PTC thermistor.
U.S. Pat. Nos. 5,945,034, 6,143,206, 6,299,801 and 6,452,476 disclose polymeric PTC thermistors comprising polymer matrix, low-molecular weight organic compound and conductive metal particles dispersed in the polymer matrix, wherein the melting point of the low molecular weight organic compound is 40-100° C. U.S. Pat. No. 6,607,679 discloses a polymeric PTC thermistor having a low-molecular weight organic compound such as wax, fat or oil. The PTC effect of all the aforesaid patents is attributed to the polymer matrix, and does not disclose or teach the way to induce PTC effect by melting and expansion of low-molecular weight organic compound nor by narrowing melting temperature distribution range of the low-molecular weight organic compound, i.e., increasing the concentration of melting point, to acquire large hold current and low trip temperature of the PTC thermistor.
U.S. Pat. No. 8,525,636 discloses a polymeric PTC thermistor comprising a polymer matrix and high conductive ceramic particles dispersed therein. The hold current per unit area (hold current/covered area) of the PTC thermistor at 60° C. is about 0.16-0.8 A/mm2, and the hold current at 60° C. is 40-95% of the hold current at 25° C. Likewise, the disclosure does not teach the way to cause PTC effect by melting and expansion of low-molecular weight organic compound nor by increasing the concentration of melting point to acquire both high hold current and low trip temperature of the PTC thermistor.